Device for measuring local radiation absorption in a body

ABSTRACT

A device for measuring the spatial distribution of radiation absorption in a body wherein a multiplicity of radiators are regularly distributed about a circle surrounding the body, each radiator emitting a wedge-shaped beam of radiation in the plane of the circle toward a different arc portion of the circle beween two other radiators, a multiplicity of adjoining detectors in each arc portion measuring radiation from the radiator emitting radiation to that arc portion, the spatial distribution of radiation absorption being calculated from the measured radiation values of all the detectors.

The invention relates to a device for measuring the spatial distributionof radiation absorption in a body, wherein the radiation to be emittedby at least one radiator is measured in a large number of directions bymeans of a number of detectors which are successively arranged in oneplane in the body, each detector having only a comparatively smalleffective measuring field, the spatial distribution being calculatedfrom these measuring values.

A device of this kind is known, for example from GermanOffenlegungsschrift 1,941,433. Therein, the radiator/detector system isdisplaced perpendicular to the radiation direction, and the absorptionis measured in a large number of directly adjacent points. Subsequently,the radiator/detector system is rotated through a given angle, afterwhich the operation is repeated, etc. The time required for obtainingthe necessary measuring values in this apparatus amounts to a fewminutes, so that using this apparatus only bodies or body parts can beexamined which can be kept completely immobile, as otherwisedisturbances occur due to lack of focus.

It is already known to reduce this measuring time (GermanOffenlegungsschrift 1,941,433, FIG. 7) by measuring the radiationemitted by the radiator by means of a plurality of detectors which arearranged in a arc of a circle around the radiator, each detectormeasuring the radiation stopped by a collimator associated with thedetector. In order to measure the absorption in the parts which are notirradiated during a first measuring series, the detectors and theassociated collimators are translated until all regions of the body tobe examined and which have not yet been irradiated have been covered.Subsequently, the radiator and the associated collimators and detectorsare rotated through a given angle, and simultaneously the detectors aretranslated back, after which the operation is repeated. The completemeasurement, however, is still comparatively time-consuming in thisapparatus. Moreover, always only a very small part of the radiationemitted by the radiator is used for the measurement (i.e. the radiationwhich is stopped by the collimators), so that when use is made of anX-ray tube as the radiator, this X-ray tube must be operated to thelimit of loadability. This has an adverse effect on the service life ofthe X-ray tube and on the reliability of the apparatus.

The invention has for its object to provide a device for measuring thespatial distribution of the absorption of radiation in a body wherebythe measuring time is reduced and in which the power of the radiator ismore efficiently used.

To this end, a device of the kind set forth according to the inventionis characterized in that the detectors are arranged in a row such thattheir effective measuring fields directly adjoin each other and coverthe entire, approximately wedge-shaped stopped radiation of theradiator.

The invention will be described in detail hereinafter with reference tothe drawing.

FIG. 1 is a diagrammatic view of a device according to the invention.

FIG. 2 shows such a device comprising various radiators arranged in acircle.

FIG. 1 shows a radiator 1 which can contain an X-ray tube orradio-active isotope. A collimator 2 passes a wedge-like beam 7 ofemitted radiation, the outer limits of said beam being denoted by 3 and3'. The beam 7 irradiates a body 4 to be examined. Behind the body 4, alarge number of detectors 5 are arranged in a circle around the radiator1 such that their effective measuring fields adjoin each other in acontacting manner, with the result that the overall radiation wedge 3,3'is measured by the individual detectors. Using this device, theabsorption in the directions determined by the connecting lines betweenthe radiator and the detectors 5 can all be measured in one operation.So as to obtain the absorption distribution in the other directions, theradiator/detector system is preferably continuously rotated about thebody 4, the radiation source being switched on and the absorption beingmeasured in given angular positions. Because the absorption in the planeof examination can be determined by way of a single measurement in oneposition of the radiator/detector system, a substantial reduction of theexamination time (to a few seconds) is feasible.

A scattered radiation diaphragm 6 which is arranged in front of thedetectors 5 and a collimator (not shown in the drawing) which limits thebeam in the direction perpendicular to the plane of the drawing suchthat only the detectors are struck by the radiation, ensure that thescattered radiation density in the region of the detectors remainssubstantially smaller than in the case of conventional tomography. Thescattered radiation density can be further reduced by arranging thedetectors not directly behind the body, but rather at a larger distancetherefrom, so that the opening angle as regards the scattered radiationcentres in the body is reduced. In that case the geometrical lack offocus is increased, but this effect can be neglected, because in thedevice according to the invention--like in the knowndevices--substantially reduced spatial resolution is used. Inparticularly critical cases, the scattered radiation can be reduced bymeans of an additional diaphragm device to be provided between thecollimator 2 and the radiator 1, if the said diaphragm opening isproportioned such that each time only a beam corresponding to theeffective measuring field of one or only a few detectors is stopped, thediaphragm device being rotatable about the radiator such that during onerotation all detectors 5 are successively irradiated. The required timeis thus increased, but the measuring time can still be reduced, becausein such a device the moving mass is essentially smaller that in theknown devices.

The detectors 5 may comprise radiation-sensitive PbO-crystallites orHgI₂ -crystals, or use can alternatively be made of radiation-sensitivesemiconductor detectors. For example, the use of light-sensitivephoto-cells or photodetectors (photo-diodes, photo-fieldeffecttransistors, etc.) is possible, these devices being preceded by anamplifier foil for converting X-radiation into visible light. Thedetectors can be operated such that they supply a signal which isproportional to the dose power (in this case pre-amplifiers which formthe time integral of the output signal of the detectors must beconnected behind the detectors) or they may be connected such that theiroutput signal is proportional to the dose, so that the subsequentpre-amplifiers only have to amplify the signal. Because the measuringvalues of all detectors are each time simultaneously released for onedirection, the subsequent computer (not elaborated herein) for measuringthe spatial distribution of the absorption in the plane usually cannotprocess the measuring values in parallel; the detector output signalsmust therefor be applied to a storage element, for example, to asample-and-hold amplifier, the outputs of all sample-and-hold amplifiersbeing connected to the computer via a multiplex device whichsuccessively applies the stored measuring values to the computer.

In addition to the row of detectors shown in FIG. 1, a further row ofdetectors may be provided in a direction perpendicular to the plane ofthe drawing, immediately adjoining the row of detectors shown. In thatcase, the radiation of the radiator must obviously be limited such thatthe two detector rows are struck by the radiation. During the subsequentprocessing of the measuring values supplied by the two rows ofdetectors, either the absorption distribution in the two planesdetermined by the detector rows can be measured or, after addition ofthe measuring values each time supplied by two adjacent detectors, themean absorption distribution for the two planes can be calculated, thesignal-to-noise ratio thus being improved.

In practice the fact must be taken into account that the intensity ofthe radiation emitted by the radiator is not uniformly distributed overthe radiation wedge 3 and that the sensitivity of the individual cellsmay differ. To this end, the gain of the pre-amplifiers connected behindthe detectors is adjusted such that, in the case of direct radiation bythe radiator (i.e. without the body 4 being in the beam path) the outputsignals are preferably the same.

In order to prevent the distribution of the absorption in the plane ofthe body to be calculated from the measuring values from beinginfluenced by temporary fluctuations in the radiation intensity, anadditional detector can be provided which is arranged above or below theplane of the drawing, so that the radiation measured thereby on the onehand is not attenuated by the body 4, to be examined, and the detectoron the other hand does not influence the radiation measured by the otherdetectors. To this end, the collimator arranged in front of the radiatormust be provided with an additional opening in the correct position. Theabsolute values of the measuring values, being dependent of temporaryfluctuations in the intensity of the radiator, should then no longer beused for the calculation, but rather the ratio between the measuringvalues of the row of detectors 5 on the one side and that of theadditional detector on the other side, this ratio being independent ofsuch fluctuations.

For the operation of the device it is important that the row ofdetectors is directed exactly to the radiation beam formed by thecollimators in front of the radiator. Alignment can be effected by meansof an optical adjusting device (not shown) which is permanentlyconnected to the radiator 1 and which emits light rays by means of amirror device, the position and the direction of the two outer lightrays corresponding to the position and the direction of the boundarylines 3,3', whilst the third light ray coincides with the bisectorbetween these two rays. The row of detectors 5 can thus be more readilydirected to the radiator. Moreover, the light rays produce light spotson the object to be examined, the layer to be measured thus beingoptically marked.

FIG. 2 shows an embodiment of the device according to the inventionwhich enables a further reduction of the measuring time. Therein, anumber of radiators (11 in the drawing) 11-21 are arranged in a circle,in the centre of which the object 4 is to be arranged, each time a rowof detectors 51-61 being associated with the said radiators,diametrically opposite in the space between two adjacent radiators.Using this device, the absorption in the plane of the body can besimultaneously or almost simultaneously measured in a number ofdirections corresponding to the number of radiators.

If all radiators were simultaneously switched on, the scatteredradiation density would be substantially increased in comparison withthe device shown in FIG. 1. So as to avoid this phenomenon, theX-radiators and the associated rows of detectors can be successivelyswitched on. This can be effected within fractions of seconds.

On the basis of spatial and economic considerations, in practice thenumber of radiator/detector systems cannot be made as high as the numberof directions in which the absorption is to be measured in the separateregions of the plane of the body to be examined. In order to measure theabsorption in all necessary directions, therefore, the system consistingof the radiators and the associated rows of detectors is rotated aboutthe body to be examined (or the body relative to the system), during onerotation through the angle 2π/n (n=number of radiators) all radiatorsbeing actuated m times, so that the absorption can be measured in m×ndifferent directions. These measurements can be performed in acomparatively short time, because the device need be rotated onlythrough a small angle and because the rotary movement can becontinuously performed. Therefore, planes of bodies which can only bevery briefly immobilized can also be examined.

What is claimed is:
 1. A device for measuring the spatial distributionof radiation absorption in a body, comprising:a multiplicity ofradiators regularly distributed about a circle of diameter sufficient tosurround said body, each radiator emitting a wedge-shaped beam ofradiation in the plane of said circle toward a different arc portion ofsaid circle between two other radiators; and a multiplicity of adjoiningdetectors in each of said different arc portions of said circle tomeasure radiation from the associated radiator emitting radiationthereto, each detector having only a comparatively small effectivemeasuring field, the spatial distribution being calculated from themeasured radiation values from said detectors.
 2. The device defined inclaim 1 wherein said circle of radiators and detectors is rotatable inthe plane thereof with respect to said body to measure radiation valuesin different rotary orientations thereof.
 3. The device defined in claim2 wherein said radiators and detectors may be successively renderedeffective. .Iadd.
 4. An apparatus for examining a body by means ofpenetrating radiation including a source means arranged to irradate thebody with a planar spread of said radiation, detector means arranged todetect the radiation to provide output signals, relating to absorptionof the radiation by the body, for processing to provide a representationof the distribution of absorption in a substantially planar section ofthe body and means adapted to scan the planar spread of radiation inrelation to the body so as to irradate said section along a plurality ofbeam paths passing through the body from a plurality of direction,wherein said detector means comprises a plurality of detector devices ofwhich a predetermined number are irradiated by said spread at any timeand wherein the scanning means is arranged to traverse the spread ofX-rays along the detector device so as to change the irradiated deviceprogressively. .Iaddend. .Iadd.5. An apparatus according to claim 4wherein the scanning means include means adapted to move said sourcemeans in relation to the body to provide at least part of the scanningof said spread of radiation. .Iaddend. .Iadd.6. An apparatus accordingto claim 4 including means adapted to move said detector means relativeto the body. .Iaddend. .Iadd.7. An apparatus according to claim 4 inwhich the scanning means include means adapted to provide at least arotational motion of said spread of radiation relative to the body..Iaddend. .Iadd.8. An apparatus according to claim 4 includingcollimator means adapted to restrict the number of said detector devicesirradiated at any time. .Iaddend. .Iadd.9. An apparatus according toclaim 8 including means for moving the collimator means in relation tothe detector means to change the detector devices so irradiated..Iaddend.